The present application is related to magnesium-lithium based alloys and methods of their preparation.
Magnesium-lithium based alloys have been recognized as having potential in aerospace applications since the early 1960's. These alloys have low density along with mechanical properties, weldability and physical properties which make them interesting for use in aircraft and aerospace applications.
Traditionally, magnesium-lithium alloys have been melted and cast by conventional methods. A problem with producing magnesium-lithium alloys in this way is that lithium and, to a lesser extent, magnesium react readily with oxygen and nitrogen in the temperature range of about 650.degree. C. to 750.degree. C. which is required for melting. In addition, the ductility of these alloys is quite sensitive to low levels of sodium impurities requiring use of high purity lithium.
One approach to protecting the molten alloys from contact with oxygen and nitrogen and the related danger of burning is to use a flux cover on the exposed surface of the melt. A second method uses an inert gas cover to protect the molten metal. A third method that has been considered is melting under vacuum. None of these methods are without problems.
Magnesium-lithium based alloys made by conventional routes have severe strength limitations. Alloys have been made containing such elements as silver, aluminum, cadmium or zinc which have high strength, but the precipitate that imparts high strength is unstable and ages excessively even at room temperature resulting in significant loss of strength. Additionally, alloys made this way are sensitive to stress corrosion cracking. Alloys which are stable at room temperature have low strength. Furthermore, these alloys all have low creep strength.
Additionally, since magnesium-lithium alloys were first seriously considered for use in aerospace and other applications, the demands on such materials have changed and there exists a need for alloys having improved properties of yield strength (under compression or tension), ultimate tensile strength, creep strength and thermal stability.
Thus, there is a need for magnesium-lithium alloys having improved characteristics that are prepared by methods which avoid the dangers associated with traditional ingot metallurgy.